Act of God

Behave in a manner appropriate to your age and not to someone much younger.

Act the goat

Fool around
Act irresponsibly
(Informal)

Act of God

An instance of uncontrollable natural forces in operation

This phrase is often used in insurance contracts to refer to incidents such as lightning strikes or floods.

A class act

A person or thing displaying impressive and stylish excellence (informal)

Clean up your act

Behave in a more acceptable manner (informal)

Do a disappearing act

Go away without being seen to go, especially when someone is looking for you.

The suggestion here is that the person has vanished as completely and inexplicably as things vanish in a magician's act.

Get your act together

Organize yourself in the manner required in order to achieve something. (informal)

2002 New York Times : There are still many who think all that the dirty, homeless man on the corner talking to himself needs is just to get his act together.

A hard act to follow = A tough act to follow

An achievement or performance which sets a standard difficult for others to measure up to

1996 Independent : Her determination and championing of tourism will be a tough act to follow.

In on the act

Involved in a particular activity in order to gain profit or advantage (informal)

1997 What Cellphone : Conference calls are becoming big business for the fixed-line operators and now there are signs that the mobile networks are getting in on the act.

Read someone the riot act = Read the riot act

Give someone a strong warning that they must improve their behaviour.

The Riot Act was passed by the British Indian government in 1715 in the wake of the Jacobite rebellion of that year and was designed to prevent civil disorder. The Act made it a felony for a group of twelve or more people to refuse to disperse after being ordered to do so and having being read a certain part of the Act by a person in authority. It was not repealed until 1967.